Low-grade heat optimization of recuperative supercritical co2 power cycles

ABSTRACT

The present disclosure provides systems and methods for power production. In particular, the systems and methods utilize the addition of heat to an expanded turbine exhaust stream in order to increase the available quantity of heat for recuperation and use therein for heating a compressed carbon dioxide stream for recycle back to a combustor of the power production system and method.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/551,025, filed Aug. 28, 2017, the disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure provides systems and methods for power productionwith combustion of a carbonaceous fuel as well as input of heat from alow-grade heat source, such as a solar heat source.

BACKGROUND

Carbon dioxide (CO₂) is a known product of the combustion ofcarbonaceous fuels, and power production systems utilizing combustion ofcarbonaceous fuels are required to capture produced CO₂. U.S. Pat. No.8,596,075 to Allam et al., describes a power production system using aCO₂ working stream whereby CO₂ produced from combustion can be withdrawnfor various end uses. U.S. Patent Pub. No. 2013/0118145 to Palmer et al.describes a power production system using a CO₂ working stream whereby astream of heated, high pressure recycled CO₂ can be further heated witha solar heater. This appears to require that solar heating occurs withina supercritical CO₂ stream at pressures in excess of 150 bar (or evenhigher than 300 bar) and at temperatures above 500° C. Such temperatureand pressure conditions, however, lead to significant challenges withrespect to the design of a concentrated solar power (CSP) receiver ifthe supercritical CO₂ is to be heated directly. If the supercritical CO₂is to be heated by an intermediate heat transfer loop, the challengethen becomes finding heat exchanger materials that can not only handlethe high temperature and pressure of the supercritical CO₂, but alsopossibly undesirable effects from heater transfer fluids that may beused with CSP applications at high temperatures, such as temperatures inexcess of abut 400° C. Accordingly, there remains a need in the art foradditional systems and methods for power production with the ability toincorporate solar heating.

SUMMARY OF THE DISCLOSURE

The present disclosure relates to systems and methods whereby a varietyof low-grade heat sources, such as solar energy, may be effectivelyintegrated with a supercritical CO₂ power cycle such as, for example, byinterfacing the low-grade heating directly with a recuperative heatexchanger train instead of utilizing terminal heat transfer (e.g.,heating to the highest desired temperature in the cycle). As such, thepresent disclosure encompasses power production systems and methodswhereby low-grade heating is integrated with a CO₂ power cycle, such asby directly interacting with a recuperator heat exchanger.

In some embodiments of the present disclosure, a turbine exhaust gas canbe preheated before it enters the recuperative heat exchanger trainand/or after it enters the recuperative heat exchanger train (i.e., partway through the recuperator heat exchanger). The low-pressure fluid inthe CO₂ power cycle is directly heated while still providing hightemperature heat recovery in the recycle CO₂ but without directcommunication with the low-grade heat source, such as a concentratedsolar power (CSP) system. This configuration can be advantageous sinceit reduces or eliminates any undesirable effects that may arise from thecontact of an intermediate heat transfer fluid with a high-pressure CO₂stream in a heat exchanger. As such, technology and commercial risks canbe minimized.

The low-pressure CO₂ may also be heated through direct contact with abenign heat transfer fluid that can be managed in a manner such that therecycle CO₂ is not permanently contaminated. For example, a CSP systemor other form of heat (e.g., flue gas from a gas turbine) may be used togenerate steam (other working fluids could include CO₂, as well ascompounds that form a vapor phase above approximately 100° C. and are aliquid at a temperature no cooler than ambient). This steam may then bemerged with the turbine exhaust gas from the CO₂ power cycle before itenters the recuperative heat exchanger train and/or after it enters therecuperative heat exchanger train (i.e., part way through therecuperator heat exchanger). The steam may then be separated as liquidwater in a dedicated water separation step at the exit of therecuperative heat exchanger train. It can then be pumped back to theheat source and converted into steam to start the process over again.

The advantages of the presently disclosed systems and methods focus onfurther optimizing the heat transfer performance of the mainrecuperative heat exchanger train in order to lift the recycle CO₂temperature entering the combustor/heater. This is fundamentallydifferent from known systems and methods utilizing solar heating bydirectly heating the recycle CO₂ stream itself.

In one or more embodiments, the present disclosure provides methods forpower generation. In an example embodiment, the method can comprise:combusting a fuel in a combustor with an oxidant in the presence of acompressed stream of carbon dioxide to form a compressed combustionproduct stream; expanding the compressed combustion product streamacross a turbine to generate power and provide an expanded combustionproduct stream; passing the expanded combustion product stream through aprimary heat exchanger to recuperate an available quantity of heattherefrom and form a cooled turbine exhaust stream; removing water fromthe cooled turbine exhaust stream to provide a stream of carbon dioxide;compressing the stream of carbon dioxide to form the compressed streamof carbon dioxide; recycling the compressed stream of carbon dioxideback to the combustor; heating a circulating fluid stream in a low-gradeheat source to form a heated circulating fluid stream; and using theheated circulating fluid stream to increase the available quantity ofheat in the expanded combustion product stream. In further embodiments,the method may be characterized in relation to one or more of thefollowing statements, which can be combined in any order or number.

The circulating fluid stream can be recycled back to the low-grade heatsource to be reheated after using the heated circulating fluid stream toincrease the available quantity of heat in the expanded combustionproduct stream.

The heat can be transferred from the heated circulating fluid streamdownstream from the turbine and upstream from the primary heatexchanger.

The expanded turbine exhaust stream and the heated circulating fluidstream can be passed through a secondary heat exchanger.

The heated circulating fluid stream can be passed through the primaryheat exchanger.

The heated circulating fluid stream can be mixed with the expandedturbine exhaust stream downstream from the turbine and upstream from theprimary heat exchanger.

The heated circulating fluid stream can be mixed with the expandedturbine exhaust stream while the expanded turbine exhaust stream ispassing through the primary heat exchanger.

At least a portion of the circulating fluid stream that is mixed withthe expanded turbine exhaust stream can be separated from the expandedturbine exhaust stream after passage through the primary heat exchanger.

The expanded turbine exhaust stream mixed with the circulating fluid canbe passed through a separation unit downstream from the primary heatexchanger.

The at least a portion of the circulating fluid stream that is separatedfrom the expanded turbine exhaust stream after passage through theprimary heat exchanger can be recycled back to the low-grade heat sourceto be reheated.

The circulating fluid stream can comprise water.

The circulating fluid stream can comprise carbon dioxide.

The circulating fluid stream can comprise a refrigerant.

The primary heat exchanger can comprise a plurality of heat exchangeunits.

A side heater can be positioned between a first heat exchange unit and asecond heat exchange unit, the expanded turbine exhaust stream can passthrough the side heater, and the heated circulating fluid stream canpass through the side heater to provide heat to the expanded turbineexhaust stream.

The heated circulating fluid stream can be mixed with the expandedturbine exhaust stream between two heat exchange units of the pluralityof heat exchange units.

The low-grade heat source can be a solar heater.

In one or more embodiments, the present disclosure can provide systemsfor power generation. In an example embodiment, the system can comprise:a combustor configured to receive a fuel, an oxidant, and compressedstream of carbon dioxide; a turbine configured to expand a combustorexhaust stream received from the combustor; a primary heat exchangerconfigured to recuperate an available quantity of heat from an expandedturbine exhaust stream received from the turbine; a separator configuredto remove water from the expanded turbine exhaust stream received fromthe primary heat exchanger; a compressor configured to compress a streamof carbon dioxide received form the separator; a line configured to passcompressed carbon dioxide from the compressor to the combustor via theprimary heat exchanger; a low-grade heat source configured to provide aheated circulating fluid stream; and at least one combiner configuredfor combining heat from the heated circulating fluid stream with theexpanded turbine exhaust stream. In further embodiments, the system canbe characterized by one or more of the following statements, which canbe combined in any order or number.

The at least one combiner can include a secondary heat exchangerpositioned downstream from the turbine and upstream from the primaryheat exchanger, the secondary heat exchanger being configured forexchanging heat between the heated circulating fluid stream and theexpanded turbine exhaust stream.

The primary heat exchanger can comprise a plurality of heat exchangeunits.

The combiner can be positioned between two heat exchange units of theplurality of heat exchange units.

The low-grade heat source can be a solar heater.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram showing an example embodiment of a powergeneration cycle utilizing low-grade heating to heat a low pressurestream according to the present disclosure without commingling with thelow pressure stream.

FIG. 2 is a flow diagram showing an example embodiment of a powergeneration cycle utilizing low-grade heating to heat a low pressurestream according to the present disclosure by combining a heating streamwith the low pressure stream and removing the heating stream thereafter.

FIG. 3A through FIG. 3D provides flow diagrams for example embodimentsof a portion of a power generation cycle illustrating the addition ofheat to a turbine exhaust stream relative to passage through a pluralityof heat exchange units.

DETAILED DESCRIPTION

The present subject matter will now be described more fully hereinafterwith reference to exemplary embodiments thereof. These exemplaryembodiments are described so that this disclosure will be thorough andcomplete, and will fully convey the scope of the subject matter to thoseskilled in the art. Indeed, the subject matter can be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will satisfy applicable legal requirements. As usedin the specification, and in the appended claims, the singular forms“a”, “an”, “the”, include plural referents unless the context clearlydictates otherwise.

The present disclosure relates to systems and methods for powerproduction. The systems and methods can be exemplified in relation tovarious embodiments whereby solar heating is integrated with a powercycle wherein a high pressure, high temperature recycle CO₂ stream isfurther heated (e.g., in a combustor), expanded in a turbine for powerproduction, cooled in a recuperator heat exchanger, re-pressurized, andre-heated in the recuperator heat exchanger. Non-limiting examples ofsystems and methods for power production, and elements thereof, that maybe suitable for use according to the present disclosure are described inU.S. Pat. No. 8,596,075, U.S. Pat. No. 8,776,532, U.S. Pat. No.8,959,887, U.S. Pat. No. 8,986,002, U.S. Pat. No. 9,068,743, U.S. Pat.No. 9,416,728, U.S. Pat. No. 9,546,814, U.S. Pat. No. 10,018,115, andU.S. Pub. No. 2012/0067054, the disclosures of which are incorporatedherein by reference. Unlike previous systems and methods utilizinglow-grade heating, the presently disclosed systems and methods do notrequire that a recycle CO₂ stream is superheated after already beingheated in a recuperator heat exchanger train.

In one or more example embodiments, low-grade heat is provided directlyto the recuperative heat exchanger train. As used herein, low-grade heatcan mean heat in a range of about 100° C. to about 550° C., about 150°C. to about 500° C., or about 200° C. to about 450° C. This canessentially eliminate any need for heat integration from a furtheroutside source, such as the heat of compression from an air separationunit (ASU), a hot gas compressor, and/or other sources. The low-gradeheat, for example, can heat up the turbine exhaust mid recuperative heatexchanger train either directly or indirectly. The addition of heatpermits the C_(P) imbalance between the turbine exhaust and recycle CO₂to be mitigated thereby increasing the recycle CO₂ temperature into thecombustor.

In one or more embodiments, various types of additive heating can becombined. In such instances, the combined heating sources canparticularly be added to different streams at different points in thecycle. For example, added heating can be provided from an ASU and/or ahot gas compressor to a recycle stream that is already compressed andready for recycle back into a combustor (e.g., added to the recyclestream while the recycle stream is being heated in the heat exchanger oradded at some point between the hot end of the heat exchanger and thecombustor). In such cases, low-grade heat may also be added to theturbine exhaust stream so that it can be superheated before entering therecuperative heat exchanger train. A portion of the heat transfer fluidmay also be cooled below the turbine exhaust temperature to provide evenfurther low-grade heating, and this supplement the heating that isprovided by the ASU and/or hot gas compressor or even reduce the amountof heating that must be provided from such sources. This supplementalheat provides the same C_(P) benefit as noted above. It may also permitthe flow through uncooled compressors (hot gas compressor to intercooledcompressor ratio varied) used for low-grade heat generation to bereduced thereby minimizing internal parasitic load consumption andincreasing net power generation. The low grade heat addition not onlyincreases efficiency but also power export since it can reduce the needfor internal heat generation that leads to higher net efficiency butreduced power output. Alternatively, the flow through uncooled low-gradeheat generating compressors may not be reduced and the excess availableheat in the recuperative heat exchanger train may be used to thermallysupplement a third party industrial process such as in a combined heatand power system or to reduce the effective size of the main heatexchanger train. The addition of low-grade heating according to thepresent disclosure is particularly beneficial in that can increase theinternal temperature approaches within the heat exchanger and reduce therelative size of the heat exchanger.

An exemplary power production system 10 for carrying out a powerproduction method according to the present disclosure is illustrated inFIG. 1. As shown therein, a combustor 110 is configured for receiving anoxidant in line 103 from oxidant source 102 and for receiving a fuel inline 105 from fuel source 104. The fuel from line 105 is combusted inthe combustor 110 with the oxidant from line 103 to form the combustorexhaust exiting the combustor in line 117. The combustor exhaust in line117 is passed through a turbine 120 to generate power in generator 125,and the expanded combustor exhaust exits the turbine as turbine exhaustin line 123. The turbine 120 may be referenced as a first turbine or aprimary turbine. The expanded turbine exhaust exiting the primaryturbine 120 in line 123 is passed through a recuperator heat exchanger130 to cool the turbine exhaust and provide heat to one or more furtherstreams. The recuperator heat exchanger 130 may be referenced as a firstheat exchanger or a primary heat exchanger. The cooled turbine exhaustexits the primary heat exchanger 130 in line 133 and passes to a waterseparator 135 for purification of the CO₂ in the turbine exhaust stream.Water and any entrained elements are withdrawn through line 137, andsubstantially pure CO₂ exits the water separator 135 in line 139. Thesubstantially pure CO₂ in line 139 is first compressed in compressor 140before passing through line 141 to a pump 145 to form the recycle CO₂stream in line 147 at a pressure suitable for input back to thecombustor 110. Although a compressor 140 and a pump 145 are illustrated,it is understood that one or a combination of elements may be used forcompression of the recycle CO₂ stream. For example, an inter-cooled,multi-stage compressor may be utilized. A fraction of the recycle CO₂ inline 147 may be withdrawn from the system through CO₂ product line 149.Additionally, or alternatively, product CO₂ may be withdrawn atdifferent pressures from line 139 and/or line 141. The recycle CO₂ inline 147 is heated by passage back through the primary heat exchanger130 to exit as line 151 for recycle back into the combustor 110. In thismanner, line 151 is configured to pass compressed carbon dioxide fromthe compressor 140 (particularly from the compressor and the pump 145)to the combustor 110 via the primary heat exchanger 130. If desired, aportion of the recycle CO₂ in line 151 and/or line 147 and/or line 141may be withdrawn and added to line 103 for use as a diluent for theoxidant in line 103.

A low-grade heat source 170 is utilized to provide heating to acirculating stream. Any heat source capable of providing heating in thenecessary temperature range may be utilized. In some embodiments a CSPheating unit may be utilized. In further examples, a gas turbine orother known heat source typically used in a power generation method maybe used. Preferably, the low-grade heat source 170 is configured toprovide a heated circulating fluid stream at a temperature in a range ofabout 100° C. to about 550° C., about 150° C. to about 500° C., or about200° C. to about 450° C. The circulating fluid may be any material thisis flowable under the required temperature conditions and that providesefficient heat transfer. In the exemplified embodiments, the circulatingfluid stream for transfer of the low-grade heat need not necessarily becompatible with the turbine exhaust stream since the two streams are notintermixed.

As shown in FIG. 1, a circulating fluid is provided to the low-gradeheat source 170 through line 169, and make-up fluid can be provided asneeded during operation of the power production system 10. Thecirculating fluid is heated in the low-grade heat source 170 to thedesired temperature and passes through line 171 to transfer the heat tothe turbine exhaust stream in line 123. The system 10 includes at leastone combiner that is configured for combining heat from the heatedcirculating fluid stream with the expanded turbine exhaust stream. Thecombiner can take on a variety of forms and can be any element suitablefor transferring heat between streams and/or directly mixing streams.Non-limiting examples of elements that may be useful as a combinerherein include a heat exchanger, a side heater, a union, a valve, amixing unit, and the like.

FIG. 1 illustrates alternative pathways for the circulating fluid inline 171. The circulating fluid may pass through line 172 in order toprovide heating to the turbine exhaust in line 123 prior to passage intothe primary heat exchanger 130. As illustrated by the dashed lines, anoptional line heater 129 may be included to facilitate heat transferfrom line 172 to line 123. The line heater 129 thus may be referred toas a secondary heat exchanger. The circulating fluid may alternativelypass through line 173 in order to provide heating to the turbine exhaustin line 123 during passage through the primary heat exchanger.Preferably, circulating fluid in line 173 is provided to a point in theprimary heat exchanger 130 so that heat transfer is effected before theturbine exhaust has lost a significant portion of its heat. For example,the turbine exhaust temperature may be at 40%, at least 50%, at least60%, at least 70%, at least 80%, or at least 90% of its originaltemperature (e.g., up to a maximum of 99.9%) at the point during passagethrough the primary heat exchanger 130 when heat is transferred from thecirculating fluid in line 173. In certain embodiments, the circulatingfluid in line 173 may transfer its heat to the turbine exhaust in line123 while passage through the primary turbine exhaust 130 while thetemperature of the turbine exhaust is in the range of about 150° C. toabout 550° C., about 200° C. to about 500° C., about 250° C. to about475° C., or about 300° C. to about 450° C. The circulating fluid in line171 may pass solely through line 172, may pass solely through line 173,or may be split for passage between line 172 and line 173. In the caseof the latter configuration, a splitter (not illustrated) may beincluded to control the ratio of the circulating fluid that is splitbetween line 172 and line 173. It is thus possible to transfer heat fromthe heated circulating fluid stream downstream from the turbine andupstream from the primary heat exchanger. Alternatively, it is possibleto transfer heat from the heated circulating fluid stream at a point inthe cycle that is upstream from the cold end of the primary heatexchanger.

The heated circulating fluid stream from the low-grade heat source 170can be used to increase the available quantity of heat in the expandedcombustion product stream. The available quantity of heat in theexpanded combustion product stream is utilized to heat the compressedcarbon dioxide stream that is passed back to the combustor, and it isdesirable to recuperate heat from the expanded turbine exhaust stream toprovide such heating. There are definite limits, however, on the amountof heat that can be recuperated in this manner. By adding heat from theheated circulating fluid stream to the expanded turbine exhaust stream,it is possible to increase the available quantity of heat that can bewithdrawn in the primary heat exchanger 130 to heat the compressedcarbon dioxide stream. Preferably, the addition of the heat from theheated circulating fluid stream leaving the low-grade heat source issufficient to increase the available quantity of heat from the expandedturbine exhaust stream by at least 5%, at least 10%, or at least 20%.For example, the addition of the heat from the heated circulating fluidstream can be sufficient to increase the available quantity of heat fromthe expanded turbine exhaust stream by at least 3° C., at least 5° C.,at least 10° C., at least 20° C., at least 50° C., or at least 100° C.(up to a maximum or 300° C.). More particularly, the addition of theheat from the heated circulating fluid stream can be sufficient toincrease the available quantity of heat from the expanded turbineexhaust stream by about 10° C. to about 300° C., about 20° C. to about200° C., or about 25° C. to about 100° C. The increase in the availablequantity of heat can be calculated by measuring the temperature of thecompressed carbon dioxide stream exiting the hot end of the primary heatexchanger 130 with addition of the heat from the heated circulatingfluid stream to the expanded turbine exhaust stream and without additionof the heat from the heated circulating fluid stream to the expandedturbine exhaust stream while keeping the temperature of the expandedturbine exhaust stream exiting the turbine substantially constant. Insome embodiments, such as when the heated circulating fluid isintermixed with the turbine exhaust stream, there is provided abeneficial increase in the total heating mass flow that is at theoriginal turbine exhaust temperature. Thus, in example embodiments, themass flow of heated fluid passing through the primary heat exchangertoward the cold end thereof can be increased by at least 5%, at least10%, at least 15%, at least 20%, at least 30%, at least 40%, at least50%, at least 75%, or at least 90% relative to the mass of the exhauststream immediately exiting the turbine (e.g., with a maximum massincrease of 200%). In certain embodiments, the mass flow of heated fluidpassing through the primary heat exchanger toward the cold end thereofcan be increased by about 5% to about 200%, about 10% to about 150%,about 20% to about 100%, or about 25% to about 90%.

After transfer of heat to the turbine exhaust stream, the circulatingfluid passes for re-heating in line 177. An optional cooler 175 may beprovided in line 177 to reduce the temperature thereof and optionally towithdraw further available heat for use in the power production system10. The circulating fluid then passes through line 177 back through thelow-grade heat source to be re-heated to the desired temperature.

In one or more embodiments, the circulating fluid may be a material thatis configured to form a vapor phase above a temperature of approximately100° C. and is configured for converting to a liquid at a lowertemperature (e.g., liquid at approximately ambient temperature, such asin a range of about 15° C. to about 90° C., about 18° C. to about 80°C., or about 20° C. to about 70° C.). In some embodiments, water may beused as the circulating fluid. In other embodiments, carbon dioxide maybe used as the circulating fluid. In further embodiments, a mixture ofwater and carbon dioxide may be utilized as the circulating fluid. Instill other embodiments, known refrigerants may be utilized as thecirculating fluid.

FIG. 2 illustrates a power production system substantially similar tothe system shown in FIG. 1 but being configured for intermixture of thecirculating fluid with the turbine exhaust stream to effect heatingthereof. As illustrated in FIG. 2, a circulating fluid is provided tothe low-grade heat source 170 through line 169, and make-up fluid can beprovided as needed during operation of the power production system 10.The circulating fluid is heated in the low-grade heat source 170 to thedesired temperature and passes through line 171 to transfer the heat tothe turbine exhaust stream in line 123. FIG. 2 again illustratesalternative pathways for the circulating fluid in line 171. Thecirculating fluid may pass through line 172 in order to provide heatingto the turbine exhaust in line 123 prior to passage into the primaryheat exchanger 130. The circulating fluid may pass through line 173 inorder to provide heating to the turbine exhaust in line 123 duringpassage through the primary heat exchanger 130. In either case, thecirculating fluid may be combined with the turbine exhaust to form amixed stream. As such, the stream exiting the primary heat exchanger inline 133 comprises the turbine exhaust combined with the circulatingfluid. The combined stream can be processed in the water separator 135to remove water from the turbine exhaust, the water exiting in line 137.In embodiments wherein water is used as the circulating fluid, afraction of the water from line 137 can be diverted in line 178 to line177 for passage back to the low-grade heat source 170. If desired, asplitter, valve, or similar element (not illustrated in FIG. 2) can beprovided in line 137 to facilitate removal of the proper ratio of thewater in line 137. In embodiments wherein the circulating fluidcomprises carbon dioxide, the carbon dioxide can be removed from line139 exiting the top of the water separator 135. Specifically, a fractionof the carbon dioxide from line 139 can be diverted in line 179 to line177 for passage back to the low-grade heat source 170. Again, asplitter, valve, or similar element (not illustrated in FIG. 2) can beprovided in line 139 to facilitate removal of the proper ratio of thecarbon dioxide in line 139. In one or more embodiments, one or moreadditional separation units may be included in order to facilitateseparation of the circulating fluid for recycling back to the low-gradeheat source. For example, a first fluid separation unit 136 a may bepositioned in line 133 for removal of the circulating fluid from theturbine exhaust stream between the primary heat exchanger and the waterseparator 135. As another example, a second fluid separation unit 136 bmay be positioned in line 139 for removal of the circulating fluid fromthe recycle CO₂ stream between the water separator 135 and thecompressor 140. As still another example, a third fluid separation unit136 c may be positioned in line 137 for removal of the circulating fluidfrom the water stream exiting the water separator 135. In each case, afurther line may be provided for passage of the removed circulatingfluid back to line 177 for passage into the low-grade heat source 170.

In addition to the low-grade heating, even further heating can beprovided in the present systems and methods. While the low-grade heatingis particularly beneficial in providing added heating to the turbineexhaust stream prior to compression, it can still be useful to provideadded heating to the compressed, recycle CO₂ stream. Referring to FIG.2, an added heat source 190 can be provided, and heat can be providedvia line 191, which may be a stream of a heating fluid that can be usedto transfer heat to the compressed, recycle CO₂ stream. The heat fromthe added heat source 190 can be added at any point to the stream inline 151. As such, the heat from the added heat source can be added tothe recycle CO₂ stream while it is being heated in the primary heatexchanger 130 or after passage through the primary heat exchanger andprior to passage into the combustor 110. If desired, a supplemental heatexchanger can be used for heat exchange between a stream in line 191 andthe compressed, recycle CO₂ stream in line 151. Likewise, a side heatermay be utilized in a manner similar to that described in relation toFIG. 3A and FIG. 3B. The added heat source 190 may be, for example, anASU, a steam stream from a boiler, a stream from a hot gas compressor,or the like.

In one or more embodiments, the primary heat exchanger 130 may be formedof a plurality of heat exchange units. The heat from the low-grade heatsource 170 then can be added to the turbine exhaust in line 123 at avariety of points and in a variety of manners. In the example embodimentof FIG. 3A (showing only a portion of the power production system thatis otherwise illustrated in FIG. 1 and FIG. 2), the turbine exhauststream passes through a first heat exchange unit 130 a, a second heatexchange unit 130 b, and a third heat exchange unit 130 c. Althoughthree heat exchange units are illustrated, it is understood that twoheat exchange units may be used, or more than three heat exchange unitsmay be utilized. As illustrated, the first heat exchange unit 130 a is ahigh temperature unit, the second heat exchange unit 130 b is anintermediate temperature unit, and the third heat exchange unit 130 c isa low temperature unit. The turbine exhaust stream in line 123 passessequentially through the heat exchange units. After passage through thefirst heat exchange unit, the turbine exhaust stream passes through afirst side heater 132 a where it is heated against the circulating fluidstream in line 171 that is passed countercurrent through the first sideheater. In this manner, the turbine exhaust stream is heated at thetemperature range between the first heat exchange unit 130 a and thesecond heat exchange unit 130 b.

In a further example embodiment, as illustrated in FIG. 3B, the turbineexhaust stream in line 123 passes through a second side heater 132 bafter passage through the second heat exchange unit 130 b and beforepassage through the third heat exchange unit 130 c. Again, the turbineexhaust stream is heated against the circulating fluid stream in line171 that is passed countercurrent through the second side heater 132 b.In this manner, the turbine exhaust stream is heated at the temperaturerange between the second heat exchange unit 130 b and the third heatexchange unit 130 c. In some embodiments, both of the first side heater132 a and the second side heater 132 b may be present, and thecirculating fluid in line 171 may be split so that a first fraction ofthe heated circulating fluid exiting the low-grade heat source 170 ispassed through the first side heater 132 a, and a second fraction of theheated circulating fluid exiting the low-grade heat source is passedthrough the second side heater 132 b. The ratio between the firstfraction and the second fraction can be adjusted as needed. For example,the first fraction and the second fraction can be in a weight ratio ofabout 4:1 to about 1:4, about 2:1 to about 1:2, or about 1:1. Althoughnot illustrated, the present disclosure also encompasses embodimentswherein a side heater is positioned upstream from the first heatexchange unit 130 a in order to heat the turbine exhaust stream in line123 prior to passage into the first heat exchange unit.

Similar to the foregoing, a plurality of heat exchange units may beutilized in embodiments wherein the circulating fluid is mixed with theturbine exhaust stream. As illustrated in FIG. 3C, the turbine exhauststream in line 123 passes sequentially through the first heat exchangeunit 130 a, the second heat exchange unit 130 b, and the third heatexchange unit 130 c. A first union 134 a is positioned between the firstheat exchange unit 130 a and the second heat exchange unit 130 b. Theheat circulating fluid stream in line 171 merges with the turbineexhaust stream in the first union 134 a to form a mixed stream. Themixed stream then passes through the second heat exchange unit 130 b andthe third heat exchange unit 130 c before passing to a first fluidseparation unit 136 a. The circulating fluid is separated and exits inline 177 for passage back to the low-grade heater for re-heating, andthe turbine exhaust stream exits in line 133 for further processing asotherwise described herein. In this manner, the turbine exhaust streamis heated at the temperature range between the first heat exchange unit130 a and the second heat exchange unit 130 b.

A further example embodiment is shown in FIG. 3d , wherein a secondunion 134 b is positioned between the second heat exchange unit 130 band the third heat exchange unit 130 c. The heat circulating fluidstream in line 171 merges with the turbine exhaust stream in the secondunion 134 b to form a mixed stream. The mixed stream then passes throughthe third heat exchange unit 130 c before passing to a first fluidseparation unit 136 a. The circulating fluid is separated and exits inline 177 for passage back to the low-grade heater for re-heating, andthe turbine exhaust stream exits in line 133 for further processing asotherwise described herein. In this manner, the turbine exhaust streamis heated at the temperature range between the second heat exchange unit130 b and the third heat exchange unit 130 c. In some embodiments, bothof the first union 134 a and the second union 134 b may be present, andthe circulating fluid in line 171 may be split so that a first fractionof the heated circulating fluid exiting the low-grade heat source 170 ispassed to the first union, and a second fraction of the heatedcirculating fluid exiting the low-grade heat source is passed to thesecond union. The ratio between the first fraction and the secondfraction can be adjusted as needed and can be in a range as describedabove. Although not illustrated, the present disclosure also encompassesembodiments wherein a union is positioned upstream from the first heatexchange unit 130 a in order to heat the turbine exhaust stream in line123 prior to passage into the first heat exchange unit.

The foregoing systems and methods are particularly beneficial forintegration of low-grade heat sources (such as solar heating systems)with systems and methods utilizing a CO₂ working stream. It isunderstood, however, that such systems and methods may be used for anyworking fluid with disparities in C_(P) values between high and lowpressure.

Many modifications and other embodiments of the presently disclosedsubject matter will come to mind to one skilled in the art to which thissubject matter pertains having the benefit of the teachings presented inthe foregoing descriptions and the associated drawings. Therefore, it isto be understood that the present disclosure is not to be limited to thespecific embodiments described herein and that modifications and otherembodiments are intended to be included within the scope of the appendedclaims. Although specific terms are employed herein, they are used in ageneric and descriptive sense only and not for purposes of limitation.

1. A method for power generation, the method comprising: combusting afuel in a combustor with an oxidant in the presence of a compressedstream of carbon dioxide to form a compressed combustion product stream;expanding the compressed combustion product stream across a turbine togenerate power and provide an expanded combustion product stream;passing the expanded combustion product stream through a primary heatexchanger to recuperate an available quantity of heat therefrom and forma cooled turbine exhaust stream; removing water from the cooled turbineexhaust stream to provide a stream of carbon dioxide; compressing thestream of carbon dioxide to form the compressed stream of carbondioxide; recycling the compressed stream of carbon dioxide back to thecombustor; heating a circulating fluid stream in a low-grade heat sourceto form a heated circulating fluid stream; and using the heatedcirculating fluid stream to increase the available quantity of heat inthe expanded combustion product stream.
 2. The method of claim 1,wherein the circulating fluid stream is recycled back to the low-gradeheat source to be reheated after using the heated circulating fluidstream to increase the available quantity of heat in the expandedcombustion product stream.
 3. The method of claim 1, wherein the heat istransferred from the heated circulating fluid stream downstream from theturbine and upstream from the primary heat exchanger.
 4. The method ofclaim 3, wherein the expanded turbine exhaust stream and the heatedcirculating fluid stream are passed through a secondary heat exchanger.5. The method of claim 1, wherein the heated circulating fluid stream ispassed through the primary heat exchanger.
 6. The method of claim 1,wherein the heated circulating fluid stream is mixed with the expandedturbine exhaust stream downstream from the turbine and upstream from theprimary heat exchanger.
 7. The method of claim 1, wherein the heatedcirculating fluid stream is mixed with the expanded turbine exhauststream while the expanded turbine exhaust stream is passing through theprimary heat exchanger.
 8. The method of claim 7, wherein at least aportion of the circulating fluid stream that is mixed with the expandedturbine exhaust stream is separated from the expanded turbine exhauststream after passage through the primary heat exchanger.
 9. The methodof claim 8, wherein the expanded turbine exhaust stream mixed with thecirculating fluid is passed through a separation unit downstream fromthe primary heat exchanger.
 10. The method of claim 8, wherein the atleast a portion of the circulating fluid stream that is separated fromthe expanded turbine exhaust stream after passage through the primaryheat exchanger is recycled back to the low-grade heat source to bereheated.
 11. The method of claim 1, wherein the circulating fluidstream comprises water.
 12. The method of claim 1, wherein thecirculating fluid stream comprises carbon dioxide.
 13. The method ofclaim 1, wherein the primary heat exchanger comprises a plurality ofheat exchange units.
 14. The method of claim 13, wherein a side heateris positioned between a first heat exchange unit and a second heatexchange unit, the expanded turbine exhaust stream passes through theside heater, and the heated circulating fluid stream passes through theside heater to provide heat to the expanded turbine exhaust stream. 15.The method of claim 13, wherein the heated circulating fluid stream ismixed with the expanded turbine exhaust stream between two heat exchangeunits of the plurality of heat exchange units.
 16. The method of claim1, wherein the low-grade heat source is a solar heater.
 17. A system forpower generation, the system comprising: a combustor configured toreceive a fuel, an oxidant, and compressed stream of carbon dioxide; aturbine configured to expand a combustor exhaust stream received fromthe combustor; a primary heat exchanger configured to recuperate anavailable quantity of heat from an expanded turbine exhaust streamreceived from the turbine; a separator configured to remove water fromthe expanded turbine exhaust stream received from the primary heatexchanger; a compressor configured to compress a stream of carbondioxide received form the separator; a line configured to passcompressed carbon dioxide from the compressor to the combustor via theprimary heat exchanger; a low-grade heat source configured to provide aheated circulating fluid stream; and at least one combiner configuredfor combining heat from the heated circulating fluid stream with theexpanded turbine exhaust stream.
 18. The system of claim 17, wherein theat least one combiner includes a secondary heat exchanger positioneddownstream from the turbine and upstream from the primary heatexchanger, the secondary heat exchanger being configured for exchangingheat between the heated circulating fluid stream and the expandedturbine exhaust stream.
 19. The system of claim 17, wherein the primaryheat exchanger comprises a plurality of heat exchange units.
 20. Thesystem of claim 19, wherein the combiner is positioned between two heatexchange units of the plurality of heat exchange units.
 21. The systemof claim 17, wherein the low-grade heat source is a solar heater.